In recent years, substantial efforts have been expended in the coating and finishing industry to reduce the organic solvent content of coating materials such as paint in order to avoid adverse environmental effects created by the solvents. Even high solids coating compositions, having a solids content of up to 60% by volume, include liquid solvent components which can escape during handling, atomization or deposition substrate and creating environmental and health hazards.
This problem has been addressed in U.S. Pat. Nos. 4,923,720; 5,106,659 and U.S. Pat. No. 5,215,253 entitled "Method and Apparatus for Forming and Dispensing Single and Multiple Phase Coating Material Containing Fluid Diluent," to Saidman et al., filed Jul. 15, 1991 which is owned by the assignee of this invention. These patents disclose coating material solutions or formulations in which the liquid solvent component is replaced to a large extent with a supercritical fluid, such as supercritical carbon dioxide, which functions as a diluent to enhance the application properties of the coating material formulation. The supercritical carbon dioxide and some liquid solvent material, e.g. about 2/3 less than is required in other coating compositions, are intermixed with polymeric pigmentary solids to form a coating material solution or formulation having a viscosity which facilitates atomization through an airless coating dispenser. As the coating material formulation is discharged from the dispensing devices toward a substrate, the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating composition and to reduce drying time of the composition on the substrate. This type of coating material formulation has the advantage of substantially reducing the adverse environmental effects caused by coating compositions having a high solvent compound.
A variety of resins, e.g. paints, can be formulated in the above described manner to reduce their solvent content, and the type of paint employed depends upon the requirements of a particular application. In certain applications, such as, for example, when rapid drying time is advantageous, "two component" paints are desirable. Two component paints formulated in accordance with the teachings of the patent mentioned above include a "high volume component" consisting of polymeric pigmentary solids, supercritical fluid and some organic solvents, and a "low volume component" such as a catalyst.
Conventionally, two component paints are formed by directing each of the components through a separate metering device, such as a metering gear pump, and then intermixing them within a static or dynamic mixer prior to discharge onto a substrate. These types of mixing systems are suitable for certain ratios and flow rates of the two components, but generally do not provide the desired accuracy at ratios greater than about 20 to 1 (high volume component to low volume component) and/or at flow rates of the low volume component less than about 20 ccm/min. Nevertheless, some two-component paints, including certain two-component coating material formulations including supercritical fluid, require a resin to catalyst ratio on the order of up to 100 to 1, and a flow rate of the low volume component or catalyst as small as 3 ccm/min. Currently available devices for intermixing such types of two component paints, at the required ratio and catalyst flow volume, are not available.